Current sources provide a fixed current, or a current dependent on some known control input (e.g., a control voltage input). An ideal current source has infinite impedance looking into either of its two terminals. Conventional practical current sources realized with transistors generally have asymmetric impedance, wherein an impedance looking into a first terminal of the current source can be orders of magnitude greater than an impedance looking into a second terminal. In some applications current sources require sourcing current from positive or negative voltage supply rails. However, voltage supply rails are inherently noisy because they are generated by active circuits. While a conventional current source may have a high impedance looking into an output terminal of the conventional current source, the asymmetric impedance of the conventional current source causes the output current of the conventional current source to be poorly isolated from voltage noise on the supply rails. Voltage supply noise transferred to the output of a conventional current source can have a magnitude several times greater than the intrinsic electronics noise of the current source itself and can therefore dominate the noise performance of a given electronic system. Thus, conventional current sources may be unsuitable for some applications requiring very low-noise operation due to feedthrough power supply noise.